The present invention relates to a new process for permanently reshaping hair, consisting in applying an inorganic bromide as a doping agent to hair, prior to or simultaneously with a reduction step using a reducing agent of the amino- or amidothiol type. The present invention also relates to a cosmetic composition containing an amino- or an amidothiol compound and an inorganic bromide for carrying out the first step of permanently reshaping hair.
The technique for permanently reshaping hair consists, in a first stage, of opening the disulphide bonds of keratin (cystins) using a composition containing a reducing agent (reduction step), and then, preferably after rinsing the hair, in reconstituting the disulphide bonds in a second stage by applying an oxidizing composition to the hair under tension (oxidation step, also termed fixing step) so as to give the hair the desired shape. This technique makes it possible to wave, uncurl or straighten hair.
The compositions for carrying out the first stage of permanently reshaping hair generally take the form of lotions, creams, gels or powders diluted in a liquid carrier, and, preferably contain a thiol as a reducing agent.
Among the latter, the products most commonly used are thioglycolic acid, glycerol monothioglycolate and cysteine.
These reducing agents, and in particular thioglycolic acid, which is generally considered to be the reference product for these compositions for permanent-reshaping, reduce the disulphide bonds of keratin effectively.
However, thioglycolic acid does not permit curling of sufficient quality to be obtained when it is used in a sufficiently basic medium, that is to say at a pH above 8.5.
Glycerol monothioglycolate, the pH optimum of which is closer to neutrality, produces inferior curling quality to that obtained with thioglycolic acid.
Although cysteine, which is generally used in the form of L-cysteine base or its hydrochloride, has the advantage of smelling considerably less unpleasant than the above two compounds, its poor reducing power does not enable curling of satisfactory intensity and hold to be obtained. Furthermore, cysteine has to be employed at a very alkaline pH (above 9.0), which influences the degradation of the hairs and increases the irritant power of the composition.
It is therefore desirable to improve the performance of conventional reducing agents while eliminating their drawbacks. The term improvement denotes both improvement in the quality of curling (e.g., the extent, hold, and beauty) obtained using the same amount of product, the ability to use a smaller amount of product and also more satisfactory conditions, such as, for example, at a pH closer to neutrality, making the treatment less degradative to hair.
Various means for improving curling hold and quality have been proposed. Among these, an increase in the exposure time, application of heat and implementation at a higher pH may be mentioned in particular. These means are effective but are unfortunately accompanied by greater degradation of hair.
It has also been proposed to use hair-swelling agents such as formamides and especially urea, alkane diols or alternatively glycol ethers, which promote penetration of the reducing agents into hair and enable some improvement to be obtained in the performance of the reducing agents, but this improvement is very inadequate in relation to amino- and amidothiols and their derivatives such as cysteine.
It has now been found most surprisingly that, when the keratin-reducing agents of the amino- or amidothiol type are combined with an inorganic bromide in well-defined proportions, it is possible to obtain a doping effect on the reducing agent which manifests itself, on the one hand in a large increase in curling hold and intensity, and on the otherhand in the possibility of employing at a neutral pH amino- or amidothiols which normally function at alkaline pH. This improvement in properties is not accompanied by any further degradation of the state of the hair fiber, which is consequently very satisfactory from the cosmetic standpoint.